Evaporated metal contacts for the fabrication of silicon carbide devices



United. States Patent O F US. Cl. 29-589 16 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of providing a thin film electrical contact for silicon carbide devices comprising electrical contact metals which have very little afiinity for wetting the surfaces of silicon carbide. A layer of a first metal which is capable of wetting the surface of silicon carbide is first disposed on the device. Then a layer of the electrical contact metal which is not capable of good wetting of the silicon carbide is next disposed on the first metal layer. A subsequent heat treatment alloys the first metal in part with the silicon carbide and in part with the second metal. The result is a thin film ohmic electrical contact affixed to the silicon carbide device.

BACKGROUND OF THE INVENTION Field of the invention This invention refers to evaporated metal contacts for silicon carbide devices.

Description of the prior art Gold, aluminum and gold-aluminum alloys have a very little tendency to wet silicon carbide. Therefore when one of these metals is deposited on a surface of a silicon carbide device to be used as a very thin electrical contact, any attempt to alloy the metal to the silicon carbide results in the forming of small beads of metal rather than the unbroken, relatively uniform film that would be expected with good wetting.

SUMMARY OF THE INVENTION In accordance with the present invention and in attainment of the foregoing objects, there is provided a process for affixing thin film metal electrical contacts, having a thickness no greater than 1 mil, to a body of semiconductor material comprising silicon carbide, the process comprising disposing a first layer of a first metal on a surface of the body, the metal being capable of wetting the surface, disposing a second layer of a second metal on the first metal layer, and alloying the first metal and the second metal together to form a third metal, the third metal being alloyed to the body of semiconductor material.

An object of this invention is to provide a process for alloying vapor deposited films of gold, aluminum and gold-aluminum alloys to silicon carbide devices.

Another object of this invention is to provide a process of first depositing a metal wetting agent on a silicon carbide crystal, then depositing a layer of gold, aluminum, or gold-aluminum alloy on the metal wetting agent and then alloying the two metals together and to the silicon carbide as an electrical contact.

Other objects will, in part, be obvious and will, in part, appear hereinafter.

DRAWINGS For a better understanding of the nature and objects of the present invention reference should be had to the following detailed description and drawings in which:

3,492,719 Patented Feb. 3, 1970 DESCRIPTION OF THE INVENTION A thin metal electrical contact may be afiixed to a surface of a body of semiconductor material in several ways such, for example as by employing thin metal sheet or foil and by electron bombardment, electron beam evaporation and sputtering. When the, electrical contact is 1 mil or less in thickness, thin metal sheet or foil often is economically unjustifiable if available. The preferred methods therefore become electron bombardment, electron beam evaporation and sputtering.

With reference to FIG. 1 there is shown schematically an apparatus 10 suitable for depositing a thin metal electrical contact on a surface of a body of silicon carbide semiconductor material by electrical bombardment. The apparatus 10 comprises a heater 12, a first metal source 14 and a second metal source 16.

The heater 12 comprises a material such, for example, as carbon. A thermoelectric couple 18 is inserted into the heater 12 to record the temperature of the heater 12.

The first metal source 14 comprises a suitable metal which will wet the surface of a body of silicon carbide material such, for example, as tungsten and tantalum. Tantalum is preferred because of its good aflinity for silicon and carbon and its alloying properties with gold. A lead 20 connects the first metal source 14 to a high voltage power source (not shown). A heater 22 comprising a tungsten filament heats the first metal source up to its evaporation temperature for the electrical bombardment of the metal for deposit on a suitable surface. Shields 24 protect the second metal source 16 and direct the electron bombardment of the metal 14 through an aperture 26 onto a surface 28 of a body 30' of silicon carbide semiconductor material disposed on the heater 12.

The second metal source 16 comprises a suitable electrical contact metal such, for example, as gold, aluminum and alloys of gold and aluminum.

The apparatus 10 is suitable for operation in a vacuum evaporation chamber.

With reference to FIG. 2 there is shown the body 30 of silicon carbide. The :body 30 comprises a first region 32 of semiconductivity, a second region of semiconductivity 34, a p-n junction 36, and the top surface 28. The second region 34 is much greater than the first region 32, the p-n junction 36 being a shallow junction.

The surface 28 is prepared for the deposition of metal by evaporation by such suitable means as lapping, polishing and etching.

The body 30 is placed on the heater 12 of the apparatus 10. All the heating elements of the apparatus 10 have been previously outgassed to reduce the chance of contamination. The body 30 is heated in a vacuum of 10 mm. of Hg or better to a temperature range of from 1000- C. to 1100" C. The body 30 is retained in this elevated temperature range for a period of about 5 minutes to burn off hydrocarbons and other impurities.

The body 30 is then cooled to a temperature range of from C. to 300 C. A temperature of C. has been found to be adequate.

Tantalum or tungsten is then evaporated by electron bombardment onto the surface 28 of the body 30'. The tantalum or tungsten, as a first metal source 14 is heated 3 to the temperature for evaporation by bombarding it with electrons from the heater 22.

FIG. 3 shows the body 30 after a layer 40 of tantalum or tungsten has been deposited on the surface 28. The layer 40 varies in thickness according to the amount of tantalum or tungsten actually required to alloy with the desired electrical contact metal in order to alloy the resulting metal to the body 30. Tantalum is preferred because it wets the silicon carbide surface 28 better than would tungsten and alloys readily with gold or a goldaluminum all y. An alloy of 1% tantalum and 99% gold has been found to be most suitable for affixing a thin metallic film electrical contact to a body of silicon carbide.

The voltage and amperage required for the operation of the electron bombardment of the first metal source 16-that is either tantalum or tungstendepends upon the equipment used and the setup of the apparatus. The degree of vacuum is important also. The greater the vacuum created, the easier the electron bombardment process progresses.

The thickness of the layer 40 is dependent upon the length of time that the deposition continues at any given temperature and vapor pressure.

With reference to FIG. 4, a second layer 42 of a metal is shown disposed on the first layer 40. The metal for the second layer 42 is obtained from the second metal source 16. The metal comprising the second metal source 16 is an electrically conductive metal which in itself will not satisfactorily wet the surface 28 of the body 30. The metal comprising the second metal source 16 will, however, alloy with the metal comprising the first metal source 14, the resulting alloy metal being capable of alloying to the body 30 to form the desired thin metallic film electrical contact. Suitable metals for comprising the second metal source 16 which will alloy with tantalum and tungsten when used as a first metal source 14 are gold and gold alloys, such, for example, as a goldaluminum alloy.

In disposing the second metal on the layer 40 of the first metal, the temperature of the vacuum evaporation chamber is raised until the temperature is reached where the material comprising the second metal source evaporates and is deposited on the layer 40 of the body 30.

The vacuum evaporation chamber is cooled and body 30 with the metal layers 40 and 42 disposed thereon is removed from the chamber.

To shape the electrical contact metals to a desired configuration, the excess metal of layers 40 and 42 must be removed. A suitable masking material such, for example, as a wax material commercially available for sale by the trade name of Apiezon wax, is disposed on the surface of the layer 42 and covers the metal to be retained for the electrical contact. The unmasked portion of the layers 40 and 42 are then removed by suitable means such, for example, as by chemical etching.

The excess metal of the layer 42 of gold is removed by chemical etching in a suitable solution such, for example, as aqua regia. To chemically etch away the excess metal of layer 40 when the metal is tantalum, a solution consisting of 2 parts by volume of hydrochloric acid, 2 parts by volume of hydrofluoric acid and 1 part by volume of nitric acid is employed. The body 30 is then rinsed with deionized water and dried.

As shown in FIG. 4 the layers 40 and 42 disposed on surface 28 of body 30 provide a rectifying electrical contact to the body 30 within a certain temperature limitation. A silicon carbide device, with or without the p-n junction 36, has rectification occurring at the interface occurring along the surface 28 between the layer 40 and the body 30, provided the temperature of the device does not exceed approximately 400 C. Thermal cycling of the metal electrical composite contact formed by the metal layers 40 and 42 does not alter the electrical characterislics of the composite contact provided the maximum temperature of approximately 400 C. is not exceeded. Therefore to achieve an ohmic electrical contact, a subsequent heat treatment in excess of approximately 400 C. is required.

The body 30 is then disposed in a suitable vacuum furnace such, for example, as a closed tube vacuum furnace or vacuum evaporation chamber. The body 30 is slowly heated up to the temperature at which alloying will occur between the metals of layers 40 and 42 and between the resulting alloy metal and the semiconductor material comprising the body 30. When the metals of the layers 40 and 42 begin to melt, the furnace temperature is raised from 50 to C. higher and retained for several minutes at this temperature to aid the two alloying processes which occur. Care is exercised to prevent the metals from becoming too fluid since most of the metals therefore might flow off the surface 28 of the body 30. The body 30 and its alloyed contact is then cooled rapidly to room temperature.

The resulting structure after the alloying process is shown in FIG. 5. A layer 44 consisting of an alloy of the metals comprising layers 40 and 42 shown in FIG. 4 is afiixed to the surface 28 of the body 30.

To assure the formation of the metal alloy layer 44, a second layer 46 preferably of the same metal comprising the layer 40 is disposed on the layer 42 of the second metal. However, metal layers 40 and 46 need not consist of the same metals. During the alloying process step, the second layer 46 provides better assurance that the metal of the layer 42 will alloy itself principally with the metal of layer 40 instead of forming globules of metal disposed on the surface 28 of the body 30- and being most undesirable for forming an electrical metal contact to the body 30. FIG. 6 shows the structure of the body 30 and the layers 40, 42 and 46 before alloying.

The following examples are illustrative of the teachings of this invention.

Example I A body of silicon carbide semiconductor material was prepared for the affixing of a thin film electrical contact to its top surface. The body comprised a wafer of silicon carbide, 0.060 inch in diameter and 0.10 inch in thickness. The body had an n-type semiconductivity region extending from a p-n junction within the body to the top surface. A p-type semiconductivity region comprised the lower portion of the body from the p-n junction to the bottom surface.

The top surface of the body was first lapped and then etched in molten salt for 3 minutes. The body was then cleaned by rinsing in deionized water, dipped in hydrochloric acid, then followed by a dip in aqua regia and then rinsed in deionized water, followed by washing in alcohol. The body was then dried.

The body was then placed in a vacuum evaporation chamber. A vacuum of 10- mm. of Hg was established in the chamber and the body was slowly heated to 1093 C. The body was held at the elevated temperature of 1093 C. :20" C. for 5 minutes to burn off hydrocarbons and other impurities from the body.

The body was then cooled within the evaporation chamber to C. Employing the apparatus set up shown in FIG. 1, tantalum was deposited on the top surface of the body by the electron bombardment method. The high voltage source supplied the required power at the rate of 2 kv. and 80 milliamps. Electron bombardment of the surface was continued for 15 minutes.

After the electron bombardment of tantalum had been completed, the body still within the vacuum evaporation chamber, was then heated to 1100 C. and gold was deposited upon the layer of tantalum previously deposited.

The body was cooled to room temperature, the vacuum broken and the body removed from the chamber. Apiezon wax, 2 mils in thickness was applied to a selected portion of the surface of the gold layer to protect what was 5 to be the thin film electrical contact to the body of silicon carbide.

The unwanted portion of the gold layer was removed by etching the gold in aqua regia. The excess portion of the tantalum layer was removed by etching the tantalum in a solution consisting of 2 parts by volume of hydrochloric acid and 1 part by volume of nitric acid.

After etching away the excess metal of the gold and tantalum layers, the Apiezon wax was removed by placing the body in boiling trichloroethylene.

After cleaning and drying, the body was cycled between room temperature and approximately 400 C. Electrical checks were made periodically and it was discovered that up to approximately 400 C. the thin film electrical contact was in actuality a rectifying electrical contact.

The body was sectioned, mounted and polished. Examination under a metallurgical .microscope showed a distinct layer of gold on a layer of tantalum which in turn was on the body of silicon carbide.

The layer of tantalum was 0.03 micron in thickness and the layer of gold was 3 microns in thickness.

Example II A body of silicon carbide was prepared in exactly the same way as the body in Example I. After the configuration of the electrical contact *was established, the body was again placed in a vacuum evaporation chamber. A vacuum of l mm. of Hg was established and the body was slowly heated to 1100 C. The gold layer showed some indication of wanting to ball up indicating that complete wetting was not taking place.

Upon reaching 1100 C., the body was heated quickly to 1185 C. and kept at this elevated temperature for 3 minutes. The body was immediately cooled to room temperature.

An electrical check indicated that the thin film electrical contact was in good ohmic electrical contact with the body of silicon carbide.

An examination of the microstructure of the body showed the metals had apparently alloyed themselves together and in turn the resulting metal alloyed had alloyed itself to the silicon carbide in the region of the thin film electrical contact. The process and time of deposition of the tantalum and gold layers was calculated to produce an alloy of 1% by weight of tntalum and the balance gold.

Example 111 A body of silicon carbide was prepared and processed in the same .manner as the body of Example I except that an additional layer of tantalum was disposed upon the layer of gold by the electron bombardment process for 4 minutes. Again the power input was 2 kv. and 80 milliamps.

The resulting body and thin film metal electrical contact was cycled between room temperature and approximately 400 C. The thin film metal electrical contact was a rectifying electrical contact as indicated by the electrical checks made.

A study of the microstructure of the body showed distinct separate layers of tantalum, gold and tantalum disposed on the top surface of the body of silicon carbide.

Example IV An examination of the microstructure of the body showed that the two layers of tantalum and the gold were apparently alloyed together, the alloy in turn being alloyed with the silicon carbide of the body in the immediate area of the contact.

The metal process steps were calculated to produce a tantalum-gold alloy having an approximate composition of 1% by weight of tantalum and the remainder gold.

During the alloying process step a visual check of the metal layers showed no indication of the gold desiring to ball up.

Further practicing of the teachings of this invention in which tungsten was substituted for the tantalum and aluminum and aluminum-gold alloys were substituted for gold, prodced thin film metal electrical contacts, being either of the rectification or the ohmic type electrical contact, suitable for use as an electrical contact for aflixing to a body of silicon carbide.

While the invention has been described with reference to particular embodiments and examples, it will be understood, of course, that modifications, substitutions and the like may be made herein without departing from its scope.

We claim as our invention:

1. A process for afiixing thin film metal electrical contacts having a thickness of no more than 1 mil to a body of semiconductor material comprising silicon carbide comprising (l) applying a first evaporated layer of a first metal on a silicon carbide surface on the body, the metal being capable of wetting the silicon carbide surface on the body, (2) disposing a second evaporated layer of a second metal on the first metal layer, and (3) heating the first evaporated layer of metal and the second evaporated layer of metal to form an alloy, the alloy being further alloyed during said heating with the silicon carbide of the body of semiconductor material to provide a good bond thereto.

2. The process of claim 1 in which the first metal is selected from the group consisting of tantalum and tungstem and the second metal is selected from the group consisting of gold, aluminum and alloys of gold and aluminum.

3. The process of claim 1 in which the first and second metal layers are vapor deposited by an electron beam evaporation process.

4. The process of claim 1 in which the first metal is tantalum and the second metal is gold.

5. A process for aifixing thin film metal electrical contacts having a thickness of no more than 1 mil to a body of semiconductor material comprising silicon carbide comprising (1) vapor depositing a first layer of a first metal on a silicon carbide surface of the body, the metal being capable of wetting the silicon carbide surface of the body, (2) vapor depositing a second layer of a second metal on the first metal layer, (3) vapor depositing a third layer of a third metal on the second metal layer, and (4) heating the first evaporated layer of metal, the second evaporated layer of metal and the third evaporated layer of metal to form an alloy, the alloy being alloyed during said heating with the silicon carbide of the body of semiconductor material to provide a good bond thereto.

6. The process of claim 5 in which the first and third metals are selected from the group consisting of tantalum and tungsten and the second metal is selected from the group consisting of gold, aluminum and alloys of gold and aluminum.

7. The process of claim 5 in which the first, second and third metal layers are vapor deposited by an electron beam evaporation process.

8. The process of claim 5 in Which the first and third metals are tantalum and the second metal is gold.

9. The process of claim 1 in which the first and second metal layers are vapor deposited by an electron bombardment process.

10. The process of claim 1 in which the first and second metal layers are vapor deposited by a sputtering process.

11. The process of claim 5 in which the first, second and third metal layers are vapor deposited by an electron bombardment process.

12. The process of claim 5 in which the first, second, and third metal layers are vapor deposited by a sputter ing process.

13. The process of claim 1, wherein the heating is carried out initially up to the temperature at which the first and second metals will begin to alloy, and then increased from 50 C. to 100 C. higher to complete the alloying of the layers of metals and the alloying thereof with the silicon carbide.

14. The process of claim 13, wherein the first metal is tantalum,

the second metal is gold,

the temperature to which the layers are heated initially is 1000 C.,

the higher temperature at which alloying is completed is 1185" C., and

the higher temperature is maintained for 3 minutes.

15. The process of claim 5, wherein the heating is carried out initially up to the temperature at which the first, second and third metals will begin to alloy, and then increased from 50 C. to 100 C. higher to complete the alloying of the layers of metals and the alloying thereof with the silicon carbide.

16. The process of claim 15, wherein the first metal is tantalum,

the second metal is gold,

the third metal is tungsten,

the temperature to which the layers are heated initially is 1000 C.,

the higher temperature at which alloying is completed is 1185 C., and

the high temperature is maintained for 3 minutes.

References Cited UNITED STATES PATENTS 1,787,749 1/ 1931 Heyroth 29621 2,856,681 10/1958 Lacy 29590 X 3,075,282 l/l963 McConville. 29590 3,128,545 4/1964 Cooper 29472.7 3,200,490 8/1965 Clymer 29-590 X 3,237,286 3/1966 Ebling et a1. 2962l X 3,298,093 1/1967 Cohen 29-590 X 3,315,350 8/1967 Kent 29527 X 3,273,979 9/1966 Budnick 29473.1 X 3,368,274 2/1968 Bronet 29-473.l X 3,382,052 5/196'8 Clarke 29-4731 X 3,047,439 7/ 1962 Van Daal et al.

PAUL M. COHEN, Primary Examiner US. Cl. X.R. 

